1. Field of the Invention
The present invention relates to a method and apparatus for designing and manufacturing individualized prosthetic implants. More particularly, the field of the invention involves interactively generating customized prosthetic implants to match specific bone shapes.
2. Description of the Related Art
Medical science has developed the ability to implant devices into human bodies to replace fractured or otherwise damaged or degenerated bones. The prothesis, the artificial part which replaces or supplements the body part, used in bone joint replacement is similar to the joint parts it is replacing. Usually, one part of the prothesis is made of metal and the other part is made of plastic because metal and plastic move against each other with little friction. The metals conventionally used are stainless steel, cobalt-chromium-molybdenum alloy, cobalt-chromium-tungsten-nickel, and titanium alloys all of which are well tolerated by the body, are corrosion resistant, and structurally secure. The plastic conventionally used is ultra-high molecular weight polyethylene which is a smooth, sturdy material that resists cracking and wear. The prothesis is conventionally cemented in place with poly methyl-methacrylate to attach to the adjacent bone or bones.
Typically, a prosthesis replaces only the damaged part of the bone, and is attached to the healthy bone by first filing or scraping the healthy bone with a rasp broach before affixing the prosthesis. Thus, both the rasp and the prosthesis should have a contour generally matching the surface of the healthy bone. Further, portions of the damaged bone may be cut off previous to applying the rasp. For example, in replacing the upper portion of the femur, the femur head is first sawed off and a rasp the shape of the inner canal of the femur is applied to the inner surface of the bone before affixing the prosthetic femur stem.
Implants and their corresponding rasps are conventionally manufactured by milling the implant shape out of stock material. The implants and rasps are first profiled and finished by a milling machine which forms the shape of the implant or rasp by milling up and down a piece of stock material to rough out the general shape before finishing the surfaces. For rasps, the finished piece is further processed to form teeth across its outer surface. In such a manner, many implants and rasps may be mass produced.
Each bone in the human body has a particular contour, and many of their the shapes may be generalized and made for any individual. However, variations in human anatomy are not uniform and consequentially many individuals require customized shapes for their implants. This is particularly true for bones which have complex topographies, e.g., bones in the knee or hip joints.
Processes are known for designing and manufacturing customized implants for such individuals. U.S. Pat. Nos. 4,822,365 and 4,936,862 (both to Walker et al. and referred to as the "Walker patents"), the disclosures of which are explicitly incorporated by reference herein, describe a method for designing an implant which utilizes a data base of implant shapes and sizes and provides a generalized implant topology which is a "best fit" for the individual requiring a customized implant. The method disclosed in the Walker patents requires the use of two x-rays of the bone being replaced, for example a femur. The x-rays are used to provide the computer with data specific to the individual, and the computer then calculates an implant topology which is used to manufacture an implant having a topography corresponding to the calculated topology.
In the femur, often the upper portion of the femur proximal to the hip requires replacement. A prosthetic upper femur stem is typically used to interfit with the internal medullary canal of the femur. Consequently, two x-rays are taken of the upper portion of the femur, a front x-ray (providing a sectional view from the medial to the lateral side) and a side x-ray (providing a sectional view from the anterior to the posterior side).
To gather the data needed for the designing process of the Walker patents, the x-rays are placed on a light board and a cursor connected to a computer is used to indicate points on the outer contour of the medullary canal of the femur. Based on the points selected on that outer contour, a computer program compares the contour of the canal, as defined by points taken from the x-rays, with a data base having contours of a random sampling of known canals. Based on the data base comparison, the program generates a three dimensional topology for a suitable replacement femur stem which matches with the contour of the canal. This computer generated topology may then be used to manufacture an implant and its corresponding rasp.
The difficulty of this method relates to the complexities and inaccuracies involved in selecting and changing the data points from the x-ray. On the first x-ray, the axial center of the femur must be vertically aligned, and points on the periphery of the medullary canal must be selected with sufficient particularity to define its contour. This requires drawing a skew line through the axial center of the femur and measuring the diameter of the femur canal, then multiplying the measured diameter by the x-ray magnification factor to enter the true diameter. The two parallel lines that are defined by the straight portion of the canal are draw out until the medullary canal opens out at the upper or proximal end of the femur. The point of divergence of the canal contour from the parallel lines must be indicated, and several subsequent points must then be indicated to define the contour of the femur extending from the straight portion of the canal. Once the first x-ray is suitably processed, a similar procedure is repeated for the second x-ray.
Once all the points from both x-rays have been selected and entered into the computer, the computer checks those points to make sure that the straight portions of the canal on both x-rays are compatible. If an incompatibility error is detected, then the entire process must be repeated. However, if no incompatibility error is detected, prints may be created which represent the topology generated by the computer program. These prints are typically sent to the physician or hospital for approval of the proposed topology. If changes are required in the proposed topology, the canal points of the x-rays must be re-entered using the procedure described above before a new topology can be generated, which allows for the possibility of human error on the second entry that may result in another rejected topology.
Another potential problem with the design of an implant by the aforementioned process involves the shape of the medullary canal. The medullary canal has a generally oval shaped cross-section, which is wider in the medial/lateral (ML) view than in the anterior/posterior (AP) view. Inconsistencies between the straight portions of the medullary canals, as defined from the AP and ML x-rays, often result in femur stem implants which are larger than the medullary canal. Thus, when the rasping device is inserted into the bone before the implant, often substantial portions of the bone are gouged which weakens the bone.
Other methods are known for determining the shape and size of an implant which utilize computerized tomographic methods. Computerized Axial Tomographic (CAT) scanners include cathode ray tube (CRT) devices, nuclear magnetic resonance (NMR) devices, positron emission (PET) devices, and ultrasonic radiant energy techniques may be utilized to generate a three dimensional topology for an implant. However, CAT scanners are more expensive than x-ray machines, are less commonly available, and require that the computer which generates the topology be connected to the CAT scanner. Also, CAT scan results are more difficult to compare with design topologies than x-ray images.
What is needed is an improved apparatus and method for generating implant design topographies using x-ray images.
Also needed are improved methods of manufacturing the implant and its associated devices.